LIGO/Virgo/KAGRA S231113bw

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S231113bw during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-13 20:04:17.911 UTC (GPS time: 1383941075.911). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S231113bw is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.4e-08 Hz, or about one in 2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231113bw

The classification of the GW signal, in order of descending probability, is BBH (79%), NSBH (17%), Terrestrial (4%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassgap) is 6%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN notice about 32 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [7], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1499 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1431 +/- 426 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/userguide/.

 [1] Klimenko et al. PRD 93, 042004 (2016)
 [2] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625
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  (2023)
 [3] Aubin et al. CQG 38, 095004 (2021)
 [4] Dal Canton et al. ApJ 923, 254 (2021)
 [5] Chu et al. PRD 105, 024023 (2022)
 [6] Chatterjee et al. ApJ 896, 54 (2020)
 [7] Singer & Price PRD 93, 024013 (2016)

H. Nishikawa, S. Sugita, M. Serino, H. Hiramatsu,  Y. Kondo (AGU),
N. Kawai (RIKEN), H. Negoro, M. Nakajima (Nihon U.),
T. Mihara (RIKEN),
Y. Kawakubo (LSU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S231113bw at 2023-11-13 20:04:17
UTC (GCN 35016
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 ).

At the trigger time of S231113bw, the high-voltage of MAXI/GSC was off,
and it was turned on at T0+285 sec (+4.8 min).
The first one-orbit (92 min) scan observation with GSC after the event
covered 53%
of the 90% credible region of the bayestar skymap from 20:09:15 to
21:34:10 UTC (T0+298 to T0+5393 sec).

No significant new source was found in the region in the one-orbit
scan observation.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

P. Veres (UAH) reports on behalf of the Fermi-GBM Team:

"For S231113bw (GCN 35016
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 ) and using the initial bayestar skymap, Fermi-GBM was observing 37% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S231113bw. An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=280.7, Dec=12.6 with a radius of 67.4 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395
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 , we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard
------------------------------------
0.128 s: 1.2     2.0    4.3
1.024 s: 0.39   0.49  1.2
8.192 s: 0.097 0.15  0.35

Assuming the median luminosity distance of 1431 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft     Normal   Hard
------------------------------------
0.128s: 0.46   0.68   2.5
1.024s: 0.15   0.17   0.69
8.192s: 0.037 0.051 0.20

R. Kumar, A. Salgundi, R. Sharma, G. Waratkar, A. Suresh, Y. Wagh, A. Arya, V. Swain, D. Raman, T. Roychowdhury, V. Bhalerao (IIT Bombay), S. Barway, G. C. Anupama (IIA), R. Norboo (IAO)

We followed up the sky localisation of the gravitational wave trigger S231113bw (GCN 35016
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 ) with the 0.7m GROWTH-India Telescope (GIT). We obtained multiple 300-sec exposures in the r' filter starting at 2023-11-13T20:38:43.52 UT, 34.4 minutes after the LVK trigger. We prioritised our observations based on galaxies in the localisation region, using the NASA/IPAC Extragalactic Database (NED, Cook et al., 2023). We obtained 39 images covering 18 sq deg, with a median depth of 20.27. Based on the "bayestar.multiorder.fits,2" skymap our images have a 1.23% probability of containing the true source.

Data were processed using the GIT pipeline (Kumar et al., 2022). The pipeline performed image subtraction with the ZOGY (Zackay et al., 2016) algorithm, and resultant images were scanned to find transients. Where available, PanSTARRS images (Chambers et al., 2016) were used as references for image subtraction. In other fields (marked with ^), we looked for new point sources absent in the GSC2.3 catalog (Lasker et al., 2008). We rejected a few candidates that were coincident with field stars and minor planets. We did not find any suitable kilonova or transient candidate in our data. 

The detailed observation log is:

| Tile ID | RA (deg) | Dec (deg) | Lim Mag (AB) |

|  4573   |  77.062  |  -25.51   |     20.44    |
|  1410   |  81.537  |  -30.41   |     20.27    |
|  1854   |  81.796  |  -29.71   |     20.24    |
|  530    |  80.178  |  -31.81   |     19.94    |
|  62104  |  255.824 |   69.00   |     19.28    |
|  9803   |  69.433  |  -17.81   |     20.94    |
|  6915   |  73.605  |  -22.01   |     20.04    |
|  4115   |  79.910  |  -26.21   |     20.27    |
|  2298   |  79.639  |  -29.01   |     20.34    |
|  2745   |  77.514  |  -28.31   |     20.51    |
|  5502   |  75.469  |  -24.11   |     20.26    |
|  3653   |  77.286  |  -26.91   |     20.72    |
|  2753   |  83.903  |  -28.31   |     19.99    |
|  60221  |  246.237 |   62.70   |     19.86    |
|  5038   |  77.506  |  -24.81   |     20.74    |
|  60678  |  245.466 |   64.10   |     19.55    |
|  16286  |  65.409  |  -8.71    |     19.62    |
|  5035   |  75.184  |  -24.81   |     20.74    |
|  10293  |  69.880  |  -17.11   |     20.41    |
|  13264  |  67.764  |  -12.91   |     19.39    |
|  6441   |  73.911  |  -22.71   |     20.53    |
|  969    |  81.274  |  -31.11   |     19.91    |
|  2746   |  78.311  |  -28.31   |     20.51    |
|  22422  |  62.539  |  -0.31    |     20.08    |
|  4112   |  77.563  |  -26.21   |     20.37    |
|  1413   |  83.979  |  -30.41   |     20.40    |
|  7871   |  73.588  |  -20.61   |     20.79    |
|  968    |  80.453  |  -31.11   |     20.09    |
|  8832   |  70.746  |  -19.21   |     20.67    |
|  63242  |  270.732 |   73.90   |     18.59    |
|  6442   |  74.673  |  -22.71   |     20.66    |
|  63241^ |  268.179 |   73.91   |     18.63    |
|  4113   |  78.348  |  -26.21   |     20.70    |
|  15273  |  65.673  |  -10.11   |     21.00    |
|  4570   |  74.729  |  -25.51   |     20.53    |
|  16795  |  65.989  |  -8.01    |     19.81    |
|  63404  |  336.108 |   74.60   |     20.18    |
|  61723^ |  248.747 |   67.60   |     19.36    |
|  10296  |  72.084  |  -17.11   |     19.54    |


The magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

The GROWTH India Telescope (GIT; Kumar et al., 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

Tomas Ahumada (CIT), Anirudh Salgundi (IITB), Robert Stein (CIT), Viraj Karambelkar (CIT), Gaurav Waratkar (IITB),  Vishwajeet Swain (IITB), Theophile du Laz (CIT), Igor Andreoni (SURA), Michael Coughlin (UMN), Mansi Kasliwal (CIT), Varun Bhalerao (IITB), Simeon Reusch (DESY), Jannis Necker (DESY), Shreya Anand (CIT), Eric Bellm (UW), S.B. Cenko (UMD), D. Perley (LJMU), D. Kaplan (UWM) report on behalf of the ZTF and GROWTH collaborations:


We observed the localization region of LVC trigger S231113bw (GCN 35016
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 ) as part of routine Zwicky Transient Facility (ZTF; Graham et al., 2019; Bellm et al., 2019) survey operations. We obtained images in the g, r, and i bands beginning at 2023-11-14T03:50:13 (7.7 hours after the burst trigger time). The observations covered 9.2% of the probability enclosed in the localization region.

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023) and emgwcave (Karambelkar et al. in prep), AMPEL (Nordin et al. 2019), and ZTFReST (Andreoni & Coughlin et al., 2021). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), and apply machine learning algorithms (Mahabal et al. 2019). We require that no spatially coincident ZTF alerts were issued before the detection time of the LVC trigger. We also run forced photometry on ZTF images (Masci et al. 2019) and ATLAS images (Tonry et al. 2018, Smith et al. 2020) and require no detections before the LVC trigger.

We do not identify any sources that pass our selection criteria.

Further follow-up of this localization region will continue as part of regular survey operations.


ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA; WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). GROWTH India telescope is located at the Indian Astronomical Observatory (Hanle), operated by the Indian Institute of Astrophysics (IIA). GROWTH-India project is supported by SERB and administered by IUSSTF, under grant number IUSSTF/PIRE Program/GROWTH/2015-16 and IUCAA.

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S231113bw (GCN Circular 35016
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 ). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S231113bw

Based on posterior support from parameter estimation [1], under the assumption that the candidate S231113bw is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassgap) is 2%.

After parameter estimation by RapidPE-RIFT [3], the updated classification of the GW signal, in order of descending probability, is BBH (96%), Terrestrial (4%), NSBH (<1%), or BNS (<1%).

For the Bilby.multiorder.fits sky map, the 90% credible region is 1713 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1186 +/- 376 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/userguide/.

 [1] Ashton et al. ApJS 241, 27 (2019) and Morisaki et al. arXiv:2307.13380
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  (2023)
 [2] Chatterjee et al. ApJ 896, 54 (2020)
 [3] Rose et al. arXiv:2201.05263
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  (2022) and Pankow et al. PRD 92, 023002 (2015)